Aircraft are subject to the buildup of ice, frost and/or snow on their surfaces while the aircraft are on the ground during cold weather in the presence of relatively high humidity, rain and/or snow. Because the buildup of ice, frost and/or snow on the aircraft surfaces can adversely affect the functioning of the aircraft, such buildups should be removed from the airplane prior to takeoff.
De-icing solutions, such as those based upon ethylene glycol and water, have been used for many years to remove ice, frost and/or snow from the aircraft surfaces. These de-icing compositions can effectively remove the buildup of ice, frost and/or snow from the surfaces but since they have low viscosities, they readily flow off the aircraft surfaces and thus provide limited protection from the formation of further frost and ice or accumulation of snow on the surfaces.
Anti-icing fluids are those that will prevent the formation of frost and ice or the accumulation of snow on surfaces over extended periods of time. The use of anti-icing fluids would be particularly desirable for overnight frost protection and for use during bad weather operation to accommodate substantial delays between the time the aircraft departs a gate and takeoff.
One approach to formulating anti-icing fluids is to add thickening agents to de-icing fluids. The thickening agents are intended to increase the viscosity of the fluid and thereby reduce its tendency to run off the surfaces of the aircraft. A number of thickened fluids are commercially available in the world. These include Kilfrost Anti-icing Barrier Compound, Hoechst A. G. Aircraft De-icing Fluid 1704, and SPCA Aircraft De-icer 84. Union Carbide Corporation introduced UCON.TM. Anti-icing Fluid E, an ethylene glycol based fluid containing polyacrylate, in 1968. In 1969, Union Carbide Europe developed a propylene glycol-based product, Aircraft De-icing fluid 251, which was a non-Newtonian fluid containing a crosslinked polyacrylic acid as a thickener.
An anti-icing fluid must meet a number of criteria. One of the most important criteria is that the anti-icing fluid be readily removed from the surfaces of the aircraft during its takeoff. This ability to be released from the aircraft surfaces must exist even if no precipitation occurs after the application of the anti-icing fluid and some of the fluid evaporates. Moreover, the fluid should have unusual rheological properties in that it must have sufficient viscosity to be retained on the aircraft surface yet require little force for it to be removed from the surface. Another important consideration in providing an anti-icing fluid is that the fluid should be capable of being applied using conventional spraying devices. Since these spraying devices can subject the thickened fluid to stress, the fluid must not be subject to undue shear instability and loss of thickening properties due to the application procedures. Loss of rheological properties are often experienced during the spray application of anti-icing fluids using a cross linked polyacrylate thickener. Another concern when using thickened fluids for anti-icing agents is the stability of the fluid during storage. For example, if an anti-icing fluid tends to gel during storage, the fluid, when applied, may be too viscous for effective removal from the aircraft or, if the gelling results in phase separation, the fluid may lose viscosity and thus not provide the desired anti-icing properties.
In addition to the foregoing requirements, a good anti-icing agent should also induce no appreciable erosive or corrosive activity, particularly towards aluminum, non-ferrous metals, steel, glass and acrylic sheeting. The anti-icing fluid should also be an effective de-icer resulting in rapid and complete thawing of ice, snow and frost and it should provide a coherent liquid film after application to the aircraft surfaces, i.e., it should wet the surfaces with which it comes in contact. Most advantageously, anti-icing fluids should be environmentally acceptable, provide minimum toxicity to plants and animals, and provide little risk of flammability.
Recent tests using commercially available anti-icing fluids on a simulated segment of an aircraft wing indicate that the anti-icing fluids can result in appreciable lift loss as compared to de-icing (unthickened) fluids. Accordingly, substantial room for improvement in anti-icing fluids for aircrafts exists.
U.S. Pat. No. 2,373,727, West, et al., discloses compositions to prevent or reduce the formation and/or accretion of ice on aircraft surfaces. The compositions comprise a jelly base consisting of gelatinous material and glycol or glycerol or homologs thereof or polyglycols or polyglycerols, e.g., diethylene glycol or triethylene glycol, and from 5 to 20 percent by weight of a liquid which is immiscible with ice or water and remains liquid at temperatures down to 10.degree. F. The immiscible liquids are disclosed to be mineral, vegetable and animal oils including essential oils such as petroleum lubricating oils, castor oils and pine oils, synthetic oils, the water immiscible alcohols and similar liquids. West, et al., state that it is necessary to add a dispersing agent such as alkali metal or organic based soaps or fatty acids or oleic acid, napthanates, sulfonic acid salts, commercial soaps or monohydric alcohols. The jelly base may be prepared from any suitable material such as gelatin, glue, soap or gum which forms a heat reversible, jelly-like mass with the glycol or glycerol.
U.S. Pat. No. 4,358,389, Koenig-Lummer, et al., disclose anti-icing compositions that contain 0.05 to 1.5 percent by weight of a particular crosslinked polyacrylate, glycol, water, 0.05 to 1 percent by weight of a water insoluble component comprising a mixed base mineral oil comprising paraffin hydrocarbons and naphthenes, 0.05 to 1 percent by weight of a surface active agent comprising alkali metal alkylaryl sulfonate, corrosion inhibitor and a quantity of at least one alkaline compound selected from the group consisting of alkali metal carbonates, bicarbonates or hydroxides and amines, such that the pH of the composition is between about 7.5 and 10. Koenig-Lummer, et al., disclose at column 5, line 54, et sec.,
"In addition to the components . . . , the agent . . . can also contain appropriate additives, preferably anti-oxidants and polysaccharides (gums) in effective quantities (gums are additional thickeners). . . . It has been found that polysaccharides have an advantageous effect on the rheological properties of crosslinked polyacrylates, particularly those having viscosity values in the lower range of the viscosity limits indicated above, that is within the range from about 1000 to 5000 mpas. Preferred polysaccharides are those of the type of high molecular xanthan gum." PA1 (a) from 5% to 85% by weight based on the weight of the total composition of a glycol, and PA1 (b) from 5% to 95% by weight of water based on the weight of the total composition; PA1 a water insoluble oil comprising at least one substantially water insoluble, partially polar compound such as carboxylic acids, sulfonic acids, phosphonic acids, salts and polar esters thereof having hydrocarbyl substituents of at least about 6, often 10 to 40, e.g., 10 to 18, carbon atoms per polar group, said substantially water insoluble partially polar compound preferably being at least one of carboxylic acid having from 10 to 18 carbon atoms and hydrocarbyl-substituted polyacrylate, said hydrocarbyl substituents having about 6 to 40 carbon atoms, in an amount of from 0.1% to 2.5% by weight based on the weight of the total composition, and if at least one other water insoluble oil is present, up to 5.0% by weight based on the total weight of the composition may be water insoluble oil; and PA1 (c) from 0.05% to 10.0% by weight based on the weight of the total composition of a thickening agent selected from the group consisting of natural and artificial gums, cellulose ethers, carboxymethylcellulose and hydroxyethylcellulose, and PA1 (d) an emulsifier selected from the group consisting of anionic surfactants and non-ionic surfactants and mixtures thereof, said emulsifier being present in an amount of from 0.1 times to 100 times the concentration of the water insoluble oil, and PA1 (e) from 0.1% to 2.5% by weight of alkanolamines based on the weight of the total composition,
Koenig-Lummer, et al., disclose suitable corrosion inhibitors to include those belonging to the group comprising inorganic metal salts, alkali metal salts of fatty acids, monoalkyl amines and dialkyl amines--optionally alkoxylated--and salts thereof, alkanol amines optionally alkoxylated--and salts thereof, esters of phosphorous acid or of phosphoric acid, and triazoles.
German Patent application No. 3,143,059, published May 5, 1983, discloses an anti-icing composition using a cellulose derivative thickener to provide a solution having a viscosity of 100 to 5000 mPa. sec at 20.degree. C. The composition contains surfactant which is an alkali alkaryl sulfonate or oxyalkylated fatty alcohol, corrosion inhibitor and alkali carbonate, alkali hydroxide or amine to give a pH of 7.5 to 10. The application states that the composition may contain other thickeners, particularly polyacrylate or xanthan gum.
An anonymous disclosure in Research Disclosure, April, 1985, No. 25246, entitled "Aircraft De-icer", describes aircraft anti-icer concentrate to which water and/or glycol can be added. The anti-icer concentrate comprises water and an antifreeze as the two main components; up to 5 percent by weight of a thickener from the group comprising cellulosic derivatives such as alkyl, hydroxy alkyl and carboxy alkyl, ethers or mixed ethers of cellulose, crosslinked or non-crosslinked polyacrylates, xanthan gum, starch or starch derivatives and guar, it being possible for all except for the polyacrylates also to act as co-thickeners; an active amount of a tenside, optionally in combination with a water insoluble compound, the tenside being selected from the group comprising polyalkylene glycols, polyalkyl oxalates of alcohols, carboxylic acids, amines or amides, olefin sulfonates, fatty alcohol sulfates and alkyl benzene sulfonates, and the water insoluble compound being selected from the group comprising fatty alcohols, fatty acid esters, fatty acid alkanolamides, mineral oils, alkyl phenols and alkyl benzenes; customary corrosion inhibitors, such as, for example, carbonates, phosphates, silicates, amines, ethanolamines and benzotriazoles; and sufficient alkali that the pH value of the agent is within the desired alkaline range.
Xanthan gum has one of the best shear stability characteristics of any known polymer. Xanthan gum, however, is highly susceptible to gelling which would be undesirable in anti-icing fluids. For example, in the field of enhanced oil recovery, gelling of a xanthan gum solution can be induced easily by the use of certain metal ions for use as fracturing fluids. Hence, in order for an xanthan gum to be suitable for use in an anti-icing fluid, a fluid composition in which the xanthan gum has substantial storage stability is essential for its practical use.